Bandgap shunt regulator circuits are well known in the art. Referring to FIG. 1 there is shown a bandgap shunt regulator circuit 10 of the prior art. The circuit 10 uses bipolar transistors Q1, Q2, Q4, Q7 and Q9 to produce a stable output low voltage reference, on the order of 1.22 volts. The circuit 10 is typically used for low voltage, i.e. less than 5 volts where Zener diodes are not suitable. In the circuit 10, the emitter of transistor Q2 is larger than the emitter of transistor Q1. As an example shown in FIG. 1 the emitter of transistor Q2 is 16 times larger than the emitter of transistor Q1. As a result, transistor Q2 with the larger emitter area requires a smaller base-emitter voltage for the same current than for the transistor Q1. The delta between the base-emitter voltage of transistor Q1 and that of the transistor Q2 is amplified by a factor of about 10 and added to the base-emitter voltage of transistor Q1. The total of these two voltages add up to approximately 1.22v, which is the approximate bandgap of silicon at 0 degrees K. The circuit 10 has the benefit of the accuracy of the Vbe term which decreases at a rate of about −2 mV/C degree. However, the circuit 10 can provide its ideal voltage only at about 1.22V for low temperature coefficient, and thus is not adjustable for voltage larger than 1.22 volts.
Referring to FIG. 2, there is shown an adjustable shunt regulator circuit 20 of the prior art. In the circuit 20, the voltage applied to resistor R1 and R2 drops when the output voltage drops due to a variation of the load. This then lowers the voltage of V1, which is the output voltage divided by R1 and R2. Thus, the non-inverting input voltage of the error amplifier is also lowered, below the internal reference voltage Vref. As a result, the error amplifier produces the base voltage of transistor TR, which suppresses the collector current. This then raises the output voltage and stabilizes it. Conversely, when the output voltage rises due to a variation of the load, V1 also rises, causing the error amplifier to raise the base voltage of TR. This then increases the collector current of the transistor TR, which lowers the output voltage and stabilizes it. Thus, the circuit 20 operates to ensure that V1 is always equivalent to the internal reference voltage Vref. The circuit 20 has the advantage that the output Vout (Vout=(1+R1/R2)×Vref) is adjustable (by changing R1 and R2), from Vref to the maximum voltage of the processing technology. However, the circuit 20 suffers from the disadvantage of having additional offset error and increased power consumption because of the error amplifier.
Referring to FIG. 3 there is shown a Brokaw bandgap reference cell 30 of the prior art. The cell 30 comprises a first NPN bipolar transistor T1, and a second NPN bipolar transistor T2, with the emitter of the first transistor T1 larger than the emitter of transistor T2. A resistor R3 is connected to the emitter of the transistor T1 to the emitter of transistor T2. A resistor R4 connects resistor R3 to ground. Each of the transistors T1 and T2 also has a load: R1 and R2 respectively, connected to the collector of the transistor T1 and T2, respectively. The load may be a resistor. An error amplifier has its inputs from the collector of the transistors T1 and T2 and supplies an output which is connected to the ends of the loads R1 and R2 and also to the bases of the transistor T1 and T2. The output of the error amplifier also provides the output of the Brokaw cell 30. In operation, transistor T1 with the larger emitter area requires a smaller base-emitter voltage for the same current. The base-emitter voltage for either transistor T1 or T2 has a negative temperature coefficient i.e., it decreases with temperature. Further, the difference between the two base-emitter voltages has a positive temperature coefficient i.e., it increases with temperature. As a result, the output of the cell 30 is the sum of the base-emitter voltage difference with one of the base-emitter voltages. With proper component choices, the two opposing temperature coefficients can cancel each other exactly and the output will have no temperature dependence. However, again because an error amplifier is used in the Brokaw cell 30, it is subject to additional offset error and increased power consumption because of the error amplifier.